Age-related hearing loss (AHL, also known as presbycusis) is a universal feature of mammalian aging and is the most common sensory disorder in the elderly population. Molecular mechanisms of AHL are poorly understood, and there are currently no known preventive strategies. We have recently shown that AHL in B6 mice can be prevented by a mutation in the pro-apoptotic gene Bak, a central player in the mitochondrial pathway of apoptosis. We and others have also shown that caloric restriction (CR), the only intervention known to retard aging and extend maximum lifespan in mammals, has a marked inhibitory effect on some mouse models of AHL, and that this effect requires the mitochondrial sirtuin Sirt3. CR improves mitochondrial function, reduces oxidative stress in the cochlea, and prevents age-related cochlear apoptosis. Our central hypothesis is that mitochondrial dysfunction plays a causal role in AHL, and that CR induces Sirt3-dependent metabolic shifts in the cochlea that prevent age-related mitochondrial dysfunction and AHL. Thus, this revised application proposes experiments designed to establish the role and mechanisms of mitochondrial dysfunction in mouse models of AHL and its prevention by CR. In Specific Aim 1, we propose to determine the role of Bak-mediated mitochondrial apoptosis in AHL. Because our previous studies have been performed in B6 mice that carry a cdh23 mutation, we propose to test the role of Bak in two models of AHL that carry a wild-type cdh23 locus, B6.CAST+ahl and CBA/J. In Specific Aim 2 we propose to determine the role of and mechanism of mitochondria in the CR-mediated prevention of AHL. These studies will focus on the role of Sirt3, and its ability to increase oxidative stress resistance and block mitochondrial apoptosis in response to CR.